1. Field of the Invention
The present invention relates generally to an optical storage device, and more particularly to an optical pickup for an optical storage device.
2. Description of the Related Art
An optical disk inclusive of a magneto-optical disk has received attention as a memory medium that becomes a core in the recent rapid development of multimedia, and it is usually accommodated in a cartridge case to be provided as an optical disk cartridge for practical use. The optical disk cartridge is loaded into an optical disk drive to perform reading/writing of data from/to the optical disk by means of an optical pickup.
The optical pickup in a recent optical disk drive intended to realize size reduction is composed of a fixed optical assembly including a laser diode, a beam splitter for reflecting and transmitting a laser beam, and a photodetector for receiving reflected light from an optical disk, and a movable optical assembly including a carriage and an actuator mounted on the carriage and having an objective lens. The carriage is movable in the radial direction of the optical disk along a pair of rails by means of a voice coil motor (VCM).
A write-power laser beam emitted from the laser diode of the fixed optical assembly is first collimated by a collimator lens, next transmitted by the beam splitter, next reflected by a beam raising mirror of the actuator, and finally focused on the optical disk by the objective lens, thereby writing data onto the optical disk. On the other hand, data reading is performed by directing a read-power laser beam onto the optical disk. Reflected light from the optical disk is first collimated by the objective lens, next reflected by the beam splitter, and finally detected by the photodetector, thereby converting the detected optical signal into an electrical signal.
In such an optical pickup, it is generally required to reduce the spot size of a light beam focused on an optical disk for the purposes of high-density recording and reproduction of information. In reducing the spot size of the light beam, it is effective to increase the numerical aperture (NA) of an objective lens or shorten the wavelength of a light beam to be emitted from a light source such as a laser diode. However, if the wavelength of the light beam to be emitted from the light source is shortened, there arises a problem on compatibility between this optical disk using the light beam having the shorter wavelength and a conventional optical disk using a light beam having a longer wavelength. For example, in the case of increasing the numerical aperture (NA) of the objective lens, there is a problem that a coma due to inclination of an optical disk tends to occur. In this respect, an information recording and/or reproducing device (e.g., digital versatile disk drive (DVD drive)) using an optical disk thinner than the conventional optical disk is known. The optical disk used in this device has a transparent substrate or protective film thinner than that of the conventional optical disk (i.e., the distance from a disk surface on which light is incident and a recording surface is reduced). Further, in the case of shortening the wavelength of the light beam to be emitted from the light source, there is a problem that an aberration tends to occur even with the same thickness of an optical disk.
Thus, the recording density of an optical disk is increasing owing to the efforts made to reduce the beam spot size. However, it is difficult to rewrite all the information resources already stored on conventional disks to new optical disks. Therefore, the optical disk drive is required to include an optical pickup capable of also reading information recorded on the conventional optical disks.
An example of such an optical pickup is disclosed in Japanese Patent Laid-open No. Hei 7-182690. The optical pickup described in this publication includes a semiconductor laser as alight emitting element, a collimator lens, a beam splitter for combining/splitting light beams, an aberration adjusting lens, an objective lens, a detecting optical system, and a photodetector. The aberration adjusting lens is a concave lens for diverging a light beam directed toward the objective lens to increase a distance of beam convergence by the objective lens. The position of the aberration adjusting lens is switched by a mechanical moving mechanism according to the wavelength of a light beam to be used.
In the case of using an optical disk having a thinner substrate (protective film), the aberration adjusting lens is moved to fall outside the optical path of the light beam. The light beam emitted from the light emitting element is converted into a collimated beam by the collimator lens, and the collimated beam is next transmitted through the beam splitter to enter the objective lens. The light beam is next focused on the optical disk by the objective lens. The light beam reflected on the optical disk is converted into a collimated beam by the objective lens, and this collimated beam is next reflected by the beam splitter to pass through the detecting optical system and enter the photodetector, thereby reading information recorded on the optical disk.
In the case of using an optical disk having a thicker substrate, the aberration adjusting lens is moved to fall inside the optical path of the light beam. The light beam emitted from the light emitting element is converted into a collimated beam by the collimator lens, and the collimated beam is next transmitted through the beam splitter and the aberration adjusting lens to enter the objective lens. The light beam is next focused on the optical disk by the objective lens. The light beam reflected on the optical disk is converted into a collimated beam by the objective lens, and this collimated beam is passed through the aberration adjusting lens and next reflected by the beam splitter to pass through the detecting optical system and enter the photodetector, thereby reading information recorded on the optical disk. At this time, the distance between the aberration adjusting lens and the objective lens is maintained constant.
In this optical pickup, an aberration is corrected for the optical disks having different thicknesses by using the single objective lens. Accordingly, this optical pickup is less expensive and more convenient than the case of using two different optical pickups so designed as to respectively correspond to two kinds of optical disks having different thicknesses. However, this optical pickup requires the mechanical moving mechanism for moving the aberration adjusting lens. Further, since the distance and angle between the aberration adjusting lens and the objective lens must be maintained constant to correct for the aberration, a mechanism for moving the aberration adjusting lens to a precise position is also required, causing an increase in device cost.
Japanese Patent Laid-open No. 2000-132859 discloses another optical pickup capable of reading information recorded on two kinds of optical disks having different thicknesses by using a single objective lens. This optical pickup includes a first integrated element unit, a second integrated element unit, a collimator lens located on the back side (downstream side) of the first integrated element unit, a plano-convex lens with an aperture limiting member located on the back side of the second integrated element unit, a beam splitter for combining/splitting light beams, and an objective lens. The first integrated element unit includes a first light emitting element and a first photodetector. The second integrated element unit includes a second light emitting element and a second photodetector.
In the case of using an optical disk having a thinner transparent substrate (protective film), the light beam emitted from the first light emitting element is converted into a collimated beam by the collimator lens. This collimated beam is transmitted through the beam splitter, and is next focused on the optical disk by the objective lens. The light beam reflected on the optical disk is converted into a collimated beam by the objective lens, and this collimated beam is transmitted through the beam splitter to enter the first photodetector.
In the case of using an optical disk having a thicker substrate, the light beam emitted from the second light emitting element is converted into a light beam having outermost peripheral rays substantially parallel to the optical axis by the plano-convex lens for producing a concentric wave aberration, and this light beam is next limited in diameter to a value smaller than the aperture of the objective lens by the aperture limiting member. The light beam thus limited by the aperture limiting member is next reflected by the beam splitter, and is next focused on the optical disk by the objective lens. The light beam reflected on the optical disk is passed through the objective lens, and is next reflected by the beam splitter to enter the second photodetector. In this manner, the information recorded on each optical disk is read.
In the optical pickup described in this publication, it is not necessary to move the plano-convex lens as an aberration adjusting lens unlike the optical pickup described in Japanese Patent Laid-open No. Hei 7-182690 mentioned above. However, it is necessary to precisely set the distance and angle between the second light emitting element (laser diode) and the plano-convex lens. The plano-convex lens does not perfectly convert the light beam into a collimated beam, so that the aberration is optimally adjusted by using the objective lens. However, there arises a positioning error of the objective lens, and assembling tolerances are exacting.
Japanese Patent Laid-open No. 2000-99983 discloses another optical pickup capable of reading information recorded on two kinds of optical disks having different thicknesses. This optical pickup includes first and second light emitting elements provided on a common substrate, a parallel plane plate having a wavelength-selective film located downstream of the first and second light emitting elements, a beam splitter located downstream of the parallel plane plate, and a collimator lens located downstream of the beam splitter. A light beam reflected by the beam splitter is detected by a photodetector.
In the case of using an optical disk having a thinner transparent substrate (protective film), the light beam emitted from the first light emitting element is reflected on the front surface (wavelength-selective film) of the parallel plane plate, next transmitted through the beam splitter, and next converted into a collimated beam by the collimator lens. This collimated beam is next focused on the optical disk by the objective lens. The light beam reflected on the optical disk is converted into a collimated beam by the objective lens. This collimated beam is transmitted through the collimated lens, next reflected by the beam splitter to enter the photodetector.
In the case of using an optical disk having a thicker substrate, the light beam emitted from the second light emitting element is refracted on the front surface of the parallel plane plate, next reflected on the back surface of the parallel plane plate, and next refracted again on the front surface of the parallel plane plate to emerge therefrom. At this time, the optical path of the light beam emerged from the front surface of the parallel plane plate coincides with the optical path of the light beam emitted from the first light emitting element and reflected on the front surface of the parallel plane plate. The light beam emerged from the front surface of the parallel plane plate is transmitted through the beam splitter, and next converted into a collimated beam by the collimator lens. This collimated beam is next focused on the optical disk by the objective lens. The light beam reflected on the optical disk is converted into a collimated beam by the objective lens. This collimated beam is transmitted through the collimator lens, next reflected by the beam splitter to enter the photodetector.
In the case that the wavelengths of the light beams emitted from the first and second light emitting elements are close to each other, the light beams can be converted into collimated beams by the collimator lens. However, in the case that the wavelengths of the light beams are considerably different from each other, an aberration occurs even in focusing the light beam from the first or second light emitting element onto the optical disk by the objective lens. In some case, the aberration cannot be canceled depending upon the distance between the collimator lens and the objective lens. Unless the distance between the collimator lens and the objective lens is fixed, a signal recorded on the optical disk cannot be read.